The present invention concerns a process for producing low-carbon chromium-containing steel.
In the process for producing low-carbon chromium-containing steel such as stainless steels it is practiced to blow an oxygen-containing gas into molten steel in a refining furnace in the atmosphere for the purpose of decarburizing the molten steel to lower the carbon level therein. This process is well known as AOD process.
The decarburization refining in the atmosphere becomes inefficient when the carbon level in the molten steel becomes low because oxygen gas blown into the molten steel is not used for decarburization but oxidizes chromium and thus decarburization efficiency gradually decreases.
On this basis the applicant proposed an improved process for refining steel, which comprises the steps of decreasing pressure in the refining furnace to 20-200 Torr at the stage where carbon content is still in a relatively high level, say 0.2%, blowing only non-oxidizing gas such as Ar into the molten steel to stir the molten steel and the slag, thereby causing reaction between chromium oxides formed during the atmospheric pressure operation and the carbon in the molten steel for decarburization and partial reduction of the chromium oxides.
This process has merits of carrying out decarburization refining in a short period of time as well as decreasing consumption of expensive Argon gas, and further, improved yield of chromium.
The process, however, has a drawback that the temperature of the molten steel increases to a high level and thus, life of refractory materials in the refining furnace becomes short.
More specifically, at the stage of the atmospheric pressure operation blowing oxygen-containing gas into the molten steel causes exothermic reactions to increase the temperature of the molten steel, and at the subsequent stage of reduced pressure operation endothermic reactions of chromium oxides and the carbon in the molten steel as well as charging of additives such as deoxidizers cause temperature decrease of the molten steel.
On the other hand, it is necessary to maintain the molten steel temperature at tapping, for facilitating subsequent casting, at a certain level or certain degrees higher than the melting point of the steel.
Therefore, in practice of the above described process which comprises the atmospheric pressure operation and the subsequent reduced pressure operation it is necessary to blow excess oxygen gas in the atmospheric pressure operation to have the molten steel temperature increased in anticipation of temperature decrease during the reduced pressure operation so that the molten steel temperature may be maintained at a certain level or higher.
In this practical operation the molten steel necessarily reaches, even for a short period of time, to an extremely high temperature, and this high temperature shortens life of refractory materials of the refining furnaces.